This invention relates in general to communication systems, and more particularly, to antenna systems making use of unbalanced antennas.
Local area network (LAN) and wide area network (WAN) applications often place small dimensional requirements on antenna systems which interface with a host system such as a computer. Several LAN and WAN products are being developed to conform to dimensional standards set forth in the personal computer memory card interface association (PCMCIA) standard. These LAN and WAN products find application in host systems such as computers which communicate over a wireless network using RF transceivers accommodated by PCMCIA receptacles available in the host system.
Small dimensional requirements to which the transceivers are desired to conform often require the use of small dimensional antenna systems. A small and inexpensive type of antenna widely used is a monopole or whip antenna. This is an unbalanced antenna made up of a long, thin, radiating element or whip with an RF grounded counterpoise that essentially provides the electromagnetic equivalent to a reflection of the whip, and allows the whip to function efficiently as an antenna.
Electromagnetic interference (EMI) specifications typically require that the transceiver and the host interface of the transceiver with the host system be electromagnetically shielded. Shielding prevents undesired RF energy from the transceiver and the host interface from broadcasting and possibly interfering with other RF systems. Shielding is usually accomplished by enclosing RF components and subassemblies with a conductive material which makes electrical contact with the ground conductors of the transceiver, the host interface and the host system. The EMI shield may also provide the unbalanced antenna with the needed counterpoise.
A disadvantage of unbalanced antennas for use with a PCMCIA transceiver is that the antenna counterpoise, which is formed using the conductive enclosure of the EMI shield, conducts RF currents not only where desired on the antenna counterpoise, but also along the EMI shield of the host interface and in turn to the host. Such undesired currents may cause radio frequency interference (RFI) with the host system, and can possibly render the host system inoperative. Another disadvantage of the undesired currents conducted on other than the counterpoise is that the antenna efficiency degrades because of reduced current magnitude on the whip antenna.
One method which has been used with limited success as a solution to the RFI problem has been to adjust the length of the whip antenna. The operating mode of the of the whip antenna as controlled by adjusting the length of the antenna moves the region of highest current magnitude along the length of the whip antenna away from the antenna counterpoise. This has the effect of reducing the amount of RF current on the counterpoise and thus the EMI shielding. The effectiveness of this solution is sensitive to the host system and therefor requires modification of the antenna system to accommodate different host systems.
There remains therefore a need for a more efficient unbalanced antenna system, including one which more effectively isolates RF currents conducted on the RF ground of the antenna counterpoise from the shielding ground of the host interface.